1. FIELD OF THE INVENTION
The present invention relates to generally a device for purifying the exhaust gases discharged from an internal combustion engine and more particularly an exhaust gas purifying device of the type having an exhaust gas reactor or the like, a secondary air supply system and an air-fuel ratio senser so that the air-fuel ratio detected in the exhaust gases (to be defined below) may be controlled at an optimum ratio at which the best performance of the reactor or the like may be expected.
2. DESCRIPTION OF THE PRIOR ART
The so-called three-way catalyst, wherein a common catalyst bed carries catalysts for oxidizing carbon monoxide (CO) and hydrocarbons (HC) and reducing nitrogen oxides (NOx) in the exhaust gases, thereby purifying these pollutants, exhibits in general the pollutant purifying characteristic shown in FIG. 1. As is seen from FIG. 1, the air-fuel ratio detected in the exhaust gases must be maintained within a hatched area in FIG. 1 so that the three-way catalyst may operate at an optimum efficiency.
To this end there has been devised and demonstrated a system of the type wherein the secondary air discharged from an air pump driven by an engine is charged through a secondary air control valve into an exhaust pipe to control the air-fuel ratio in such a way that a reactor or the like may operate most efficiently. In this specification, the term "exhaust-gas air-fuel ratio" will be used and is defined by the following relation: ##EQU1## The intake air and fuel are mixed in a carburetor and charged into the cylinders of the engine, whereas the secondary air is charged into the exhaust pipe upstream of the reactor.
The secondary air control valve is a two-way valve; that is a valve for opening or closing a secondary air supply passage. However, when the initially wide opened two-way valve is being closed, the flow rate of secondary air passing therethrough remains almost unchanged and is substantially equal to the flow rate when the valve is wide opened, immediately before the valve is completely closed. As a result, an excessive quantity of secondary air is charged into the exhaust pipe so that with the prior art secondary air supply devices it is impossible to control the "exhaust-gas air-fuel ratio" with a higher degree of accuracy, and consequently the "exhaust-gas air-fuel ratio" varies over a wide range.
So far diaphragms have been widely used for controlling the two-way valves, but when the volumes of two pressure chambers defined above and below a diaphragm are different from each other, there is a difference between a time required for closing the two-way valve and a time required for opening it so that the "exhaust-gas air-fuel ratio" cannot be controlled with a higher degree of accuracy and in a quick response.